Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, a common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of cumene hydroperoxide in the presence of heterogeneous or homogeneous catalysts into equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone.
Thus, a process that coproduces a ketone other than acetone may be an attractive alternative route to the production of phenol. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
Phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. This reference also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
Although the production of phenol and cyclohexanone from cyclohexylbenzene appears to be analogous to the Hock process for producing phenol and acetone from cumene, the chemistries in each step are actually very different. For example, the chemistry of the cleavage of cyclohexylbenzene hydroperoxide is much more complicated than that for cumene hydroperoxide and more by-products (both in types and amounts) can form. Thus, cleavage of cyclohexylbenzene hydroperoxide to phenol and cyclohexanone is acid catalyzed and, although a variety of acid catalysts can be used, sulfuric acid is preferred for its low cost and easy availability. However, significant yield loss to by-products (both primary and secondary) can occur in the sulfuric acid-based cleavage of cyclohexylbenzene hydroperoxide. Typical primary by-products include the β-scission products such as hexanophenone and 6-hydroxylhexanophenone (6-HHP). Examples of secondary by-products include those derived from cyclohexanone, such as 2-(1-cyclohexenyl)cylohexanone and 2-(cyclohexylidene)cyclohexanone (cyclohexanone aldol condensation products), 2-hydroxycyclohexanone and cyclohexenone (cyclohexanone oxidation products). Formation of the primary by-products results in loss of both phenol and cyclohexanone; while secondary by-products further reduce yield to cyclohexanone.
There is therefore significant interest in developing an acid-catalyzed process for the cleavage of cyclohexylbenzene hydroperoxide in which the yield of phenol and cyclohexanone is maximized. According to the invention, it has now been found that achieving high yields of phenol and cyclohexanone in the conversion of cyclohexylbenzene hydroperoxide in the presence of an acid catalyst is dependent not only on the composition of the cleavage reaction medium but also on the ratio of mixing rate to the reaction rate of the reaction components. In particular, it has been found that improved reaction selectivity is achieved when the ratio of tR/tM≧10, where tR is the half-life of cyclohexylbenzene hydroperoxide under the cleavage conditions employed and tM is a characteristic mixing time for the reaction components under the mixing conditions employed. The time tM is determined in a separate calibration test by injecting a tracer material into the reaction components and measuring the time under the mixing conditions employed in the cleavage process for at least 95% by volume of the entire reaction medium to reach at least 95% of the volume-averaged tracer material concentration.